The permeability characteristics of the rat choroidal capillaries have been studied using hemeproteins as tracers. Smaller molecules such as horseradish peroxidase readily permeated the capillary endothelium and crossed Bruch's membrane; larger tracers such as hemoglobin and lactoperoxidase were significantly restricted at the level of the diaphragmed fenestrae and catalase and ferritin were virtually excluded from entering Bruch's membrane, even after long periods of circulation. The evidence indicates that the choriocapillary endothelium is a significant barrier to movement of large molecules. Using cationic ruthenium red, anionic sites have been demonstrated in Bruch's membrane, where they appear in rows along the two basal laminae. Sites associated with the capillary endothelium were removed by prior exposure to chondroitin ABC while those associated with the pigment epithelium were removed by exposure to crude heparinase. The findings indicate that the anionic sites consist, at least in part, of chrondroitin sulfates and heparin sulfate. Anionic sites have also been visualized in rabbits with highly cationic polymer, polyethylenemine. Studies on the immunocytochemical localization of albumin and IgG, using peroxidase-labeled, Fab fragments of antibodies to these proteins are in progress. The permeability of the rat choriocapillaries to injected hemeprotein tracers at various stages of development are underway. Preliminary findings indicate a restriction to molecular movement at the capillary endothelium at certain times during postnatal development.